Most radios operate in multipath environments. In such multipath environments, more than one transmission path exists between the transmitter and receiver. Narrowband radios suffer in multipath environments due to frequency selective fading, which is caused by the different time delays on the various paths and the destructive combining of the signal from all the paths. Narrowband radios can employ rake receiver structures to combine signals from the multiple paths, but this is a difficult and expensive process since narrowband systems lack the time-domain resolution to easily resolve the multipath terms. Rake is a term used to describe the coherent combining of energy from a plurality of multi-path induced replicas of the desired signal.
By definition, however, ultrawide bandwidth (UWB) systems have high time-domain resolution, and thus can resolve multipath signals. High chipping rate UWB systems have the advantage of operating in quasi-stationary multipath environments where the multipath is changing much slower than the code duration.
A raking receiver is thus used when multiple paths exist between two radios. FIG. 1 is a block diagram of a wireless system having two radios in which there are multiple transmission paths between the two radios.
As shown in FIG. 1, the wireless system 100 includes first and second radios 110 and 120, having first and second antennas 115, 125, respectively. There is a direct line of sight path 140 between the two radios 110 and 120, but there are also indirect paths 150 and 155 caused by bouncing signals off of other objects 130, 135 in the area around the two radios 110 and 120.
As a result, if the first radio 110 sends a wavelet out of the first antenna 115, the second antenna 125 will receive a plurality of wavelets having an arbitrary spacing that correspond to that signal as it passes along one direct path signal 140 and multiple different reflected paths 150 and 155. And although FIG. 1 shows only two reflected signals 150 and 155 bouncing off of two objects 130 and 135, there can be many more reflections off of multiple other objects. In rooms you can have hundreds, even thousands, of reflections with all kinds of different reflected path lengths.
Furthermore, depending on the properties of each object 130, 135, the strongest signal received at the second antenna 125 may be a reflected signal 150, 155 rather than the direct signal 140. One reason for this is that there could be something collecting energy at one of the objects 130, 135 and focusing it towards the receiving antenna 125. Another reason that a reflected signal may stronger than a direct signal is that there could be multiple objects that cause reflections having the same reflected path length. For example, if path 150 has a length L1, path 155 has a length L2, and L2=L2, the path lengths will be exactly matched. Ma result of this, one wavelet will travel from the first antenna 115 along path 150 to the second antenna 125, and another wavelet will travel from the first antenna 115 along path 155 to the second antenna. But since the path lengths are the same, both wavelets will arrive at the second antenna 125 at the same time and they would add their strengths together. Therefore it's not necessary that the shortest path signal be the strongest one received at the receiver.
FIGS. 2A-2C are graphs showing examples of the strengths of received signals in a multipath environment. In particular, FIGS. 2A-2C show the strengths of signals received at the second antenna 125 when a single wavelet is output from the first antenna 115 and travels only along the three paths 140, 150, and 155 of FIG. 1.
As shown in FIG. 2A, three wavelets 205, 210, and 215 arrive when the paths 140, 150, and 155 are of different length and the signal strengths are about the same size. FIG. 2B shows three wavelets coming in 220, 225, and 230 where the paths 140, 150, and 155 are of different length and the signal strength of one path is much larger that the other paths. As a result, one of the wavelets 230 is larger than the other two. FIG. 2C shows only two wavelets 240 and 245 being received because the wavelets from the two reflection paths 150 and 155 have the same path length (i.e., L1=L2). As a result, the two reflected wavelets add their strength and so the second wavelet 245 in this instance is larger than the first wavelet 240 from the direct path 140.